Oxynitride film and its manufacturing method

ABSTRACT

The oxynitride film according to the present invention contains Ga and/or Al and has O/N ratio of at least 0.15. This film is obtained by relying on, for example, chemical vapor deposition technique. The O/N ratio in the film may be varied by, for example, varying the distance between the substrate and the substance-supply source, or by varying the proportion of an oxidizing gas contained in a carrier gas. This film is used either as a surface passivation film of III-V compound semiconductors such as GaAs, or as an insulating film for active surface portions of IG-FET, or as an optical anti-reflective film.

This is a continuation of application Ser. No. 23,766, filed Mar. 26,1979 and now abandoned.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention concerns an oxynitride film containing galliumand/or aluminum, and its manufacturing method.

Such oxynitride film as stated above is very useful and effective foruse not only as an optical but also an electrical material because it ispossible to continuously vary the values of, for example, refractiveindex and dielectric constant of this film by varying the proportionbetween oxygen and nitrogen which are contained in the film to beformed. Furthermore, by depositing such oxynitride film on asemiconductor surface, it is possible to use it as a surface passivationfilm thereof also. Here, the term "surface passivation film" means notonly a surface-protection film in its narrow sense, but also agate-insulating film of such articles as insulated-gate type fieldeffect transistor, insulated-gate type static induction transistor, andinsulated-gate type semiconductor integrated circuit using thesetransistors, and furthermore the film may serve as: a selectivediffusion film employed in the planar semiconductor device manufacturingtechnique, a thin encapsulating film in post-ion-implantation annealingtechnique, a masking film employed in the selective growth technique,and a thin insulating film which is incorporated in those activeportions of an active device and an insulating film for passive useother than the use mentioned above.

(b) Description of the Prior Art

Description will hereunder be made firstly with respect to surfacepassivation of III-V compound semiconductors. Known surface passivationmethods for III-V compound semiconductors which are made typically ofGaAs may be divided roughly into the following three types.

(1) A method of using, also for III-V semiconductors, such depositedfilms as SiO₂, Si₃ N₄, Al₂ O₃ and P₂ O₅ which have been used aspassivation films for the surfaces of silicon semiconductors. This knownmethod, while having the drawback that the deposition temperature isrelatively high, is being used quite frequently at present. Especiallyan SiO₂ film is widely used in technical fields excluding thosesemiconductor devices which are designed to use their surface portionsjust beneath the interface with the SiO₂ as an active region, forreasons such as easiness of formation of this film and also in view ofthe utilization of the accumulated knowledges concerning the method ofproducing planar type silicon semiconductor devices, and for likereasons, in spite of the important drawbacks that this SiO₂ film tendsto take in Ga from the surface of a substrate made of GaAs or GaP andthat thereby it will damage the stoichiometry of the surface of thesubstrate.

(2) A method of forming, on a substrate, a native oxide filmcorresponding to a thermal oxidation film of silicon, in place of thedeposited film stated in (1) above. Among those techniques belonging tothe type categorized by this paragraph (2), the anodic oxidation methodhas the advantage that an insulating thin film can be formed at amarkedly low temperature as compared with the deposition method and alsowith the thermal oxidation method, irrespective of the instances whereina solution is used or a gas plasma is used. Conversely, however, thisanodic oxidation method has the disadvantage that it is thermallyunstable, and therefore, it has the drawback that the quality of thefilm will change substantially at a temperature below the temperaturerange adopted for thermal diffusion of impurities andpost-ion-implantation annealing. Furthermore, the interface between ananodic oxide film and a substrate made of GaAs or GaP tends to contain anumber of defects, so that when this film is utilized as an insulatingfilm of IG-FET (Insulated-Gate Type FET), there still cannot be obtainedas yet a large value of surface mobility comparable with that within thebulk, and thus at the current technical stage, it is not possible forthe anodic oxide film to fully display those advantages and features onapplying it to the surface of GaAs and GaP substrates which arerepresented by higher mobility as compared with a silicon substrate. InIII-V semiconductors which, essentially, are binary compounds, a directthermal oxidation of their surfaces has not yet produced anysatisfactory results with respect to the quality of the film produced orto the state of interface. Such native oxide film has the furtherdrawback that it is dissolved in acids such as HF, HCl, and H₂ SO₄.Therefore, native oxide films inconveniently cannot be used in suchmanufacturing process as would comprise a number of steps.

(3) A method of performing chemical oxidation by the use of, forexample, hot hydrogen peroxide solution. This method is entailed bylimitation in the thickness of oxide film which is formed, andaccordingly the extent of application of this method is limited also.

As discussed above, these known surface passivation methods for III-Vcompound semiconductors invariably have both strong points and weakpoints.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to eliminatethe drawbacks noted in conventional surface passivation methods, and toprovide new passivation films which can be formed at a relatively lowtemperature and which has a low surface state density, and also toprovide a method of manufacturing such new passivation films. Moreparticularly, the present invention contemplates the provision of newpassivation films which are expressed generally as gallium oxynitride(GaO_(x) N_(y)) films, aluminum oxynitride (AlO_(x) N_(y)) films, andmixtures thereof, i.e. gallium-aluminum oxynitride (Ga_(x),Al_(y),O_(x)N_(y)) films, to passivate the surfaces of Group III-V compoundsemiconductors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration, showing the relationship betweensurface state density N_(FB) and O/N ratio by atomic fraction in aGaO_(x) N_(y) film of the present invention.

FIG. 2 is a diagrammatic illustration, showing the relationship betweenadherency and O/N ratio by atomic fraction in said GaO_(x) N_(y) filmshown in FIG. 1.

FIG. 3 is a diagrammatic illustration showing the capacitance-voltagecharacteristic of MIS structure using a GaO_(x) N_(y) film of thepresent invention as an insulator.

FIG. 4 is a diagrammatic illustration, showing the compositiondistribution of the GaO_(x) N_(y) film in the direction of the depth ofthis film employed in FIG. 3, measured by using Auger electronspectroscopy technique combined with Argon ion-sputtering technique.

FIG. 5 is a diagrammatic illustration of the composition distribution ofan example of film according to the present invention.

FIG. 6 is a schematic illustration, showing the positional relationshipbetween the substrate and the substance-supply source placed in areaction tube for the deposition of a GaO_(x) N_(y) film on thesubstrate.

FIG. 7 is a diagrammatic illustration, showing the compositiondistribution of another example of the film according to the presentinvention.

FIG. 8 is a diagrammatic illustration showing the compositiondistribution prior to and after a heat treatment of a GaO_(x) N_(y) filmof the present invention in an NH₃ gas containing an oxidizing gas.

FIGS. 9 and 10 are diagrammatic illustrations for explaining anotherexample of formation of a GaO_(x) N_(y) films of the present invention.

FIGS. 11A to 11C are schematic sectional views, showing the instanceswherein the GaO_(x) N_(y) film of the present invention is applied to aninsulating film of insulated-gate type field effect transistors.

FIGS. 12A and 12B are schematic sectional views, showing the instanceswherein a GaO_(x) N_(y) film of the present invention is employed as asurface protection film of pn junction, respectively.

FIG. 13 is a diagrammatic illustration, showing the variation ofrelative dielectric constant exhibited by a GaO_(x) N_(y) film of thepresent invention, depending on the changes in its O/N ratio by atomicfraction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Description will hereunder be made of the principles of the surfacepassivation method of the present invention, and also on someembodiments by referring to the accompanying drawings.

For the convenience of explanation, gallium arsenide (GaAs) will betaken up as an example of III-V compound semiconductors which areintermetallic compound semiconductor substrates formed with at least oneelement of the Group III elements and at least one element of the GroupV elements in the periodic table, since GaAs is most widely employedamong III-V semiconductors. The passivation method based on theprinciples of the present invention can be also applied tomulti-component compound semiconductors containing Ga and As such asGa_(x) Al_(1-x) As, GaAs_(x) P_(1-x) and In_(x) Ga_(1-x) As_(y) P_(1-y)and also to other III-V compound semiconductors. It should be understoodhere that the oxynitride films according to the present invention can beformed by thermal decomposition technique, or by ordinary chemical vapordeposition (CVD) technique conducted at normal pressure or at a lowpressure, or by plasma CVD technique, or by reactive sputteringtechnique, or by using a method designed to vary O/N ratio by separatelycontrolling the respective arrival rates, onto the surface of asubstrate, of a nitride and an oxide from their supply sources, or by amethod of performing heat treatment in an ammonium gas containing anoxidizing gas, or by a method of performing heat treatment in a nitrogengas containing a nitrogen oxide gas, or by methods equivalent to theabove-mentioned various methods. However, as a concrete embodiment,description will hereunder be made, first of all, of a GaO_(x) N_(y)film which is formed by thermal decomposition method. O/N ratiodescribed above is based on atomic fraction throughout the descriptionof the present invention.

As the supply source of Ga and N, there is used a complex compound ofGaBr₃ and NH₃, which compound being prepared by treating galliumtribromide GaBr₃ in ammonia NH₃ gas. In order to control the compositionratio between O and N (O/N) in the film which is to be formed, GaBr₃ issubjected to absorbing a controlled amount of water or oxygen.Controlling of O/N ratio of the film to be formed may be carried out, inaddition to the above-mentioned manner, by, for example, mixing nitrogenmonoxide NO gas or oxygen O₂ gas in NH₃ as a carrier gas is which it isalso possible to use the complex supply source of GaBr₃ --NH₃ instead ofthe water-added one. The Ga, N and O-supply source thus prepared issubjected to heating at an appropriate temperature ranging between 250°C. and 350° C. to control the amount or rate of evaporation as required.Such supply source is transported onto a GaAs substrate by the aforesaidcarrier gas of NH₃ at the rate of, typically, 2 liters/min. By thermaldecomposition reaction, a GaO_(x) N_(y) film is deposited on said GaAssubstrate. The temperature of this GaAs substrate is set by means of aresistance-heating furnace or by RF (radio frequency) heating means. Incase a resistance-heating furnace is used, it is possible to finelycontrol the O/N ratio in the GaO_(x) N_(y) film which is to be formed,by varying the distance L between the supply source and the GaAssubstrate. More particularly, in case this distance L is set at a smallvalue, O/N ratio in the film will increase, whereas in case the distanceL is set at a large value, O/N ratio in the film will decrease.According to this depositing method, substantial deposition takes placetill the temperature goes down close to 400° C. However, at 400° C. therate of deposition becomes considerably small, being typically about 5A/min., and thus such deposition is far from being practical. On theother hand, at a high temperature of 600° C. or higher, the surface ofthe GaAs substrate becomes degraded, and furthermore the quality of theGaO_(x) N_(y) film which is produced will tend to contain an excessamount of Ga. Therefore, the range of temperatures which does not bringabout rejectable deterioration of the surface condition of the substrateduring deposition, and which allows the quality of the GaO_(x) N_(y)film to be kept reasonably satisfactory, and also which is practical, is450°-550° C. As an example, the result of a couple of experiments ismentioned below. By setting the temperature of the substrate at 450° C.and 500° C., the temperature of the supply source at 280° C., and theflow rate of carrier NH₃ gas at 2 liters/min., the deposition rates inthese two instances are: 20-30 A/min. and about 50 A/min., respectively.O/N ratio can be varied according to the above-mentioned manner, and therange from zero (in case of pure GaN) up to the maximum of about 1.5 canbe easily obtained. The surface state density N_(FB) of the GaO_(x)N_(y) - GaAs system thus produced depends on O/N ratio, i.e. the smallerthe O/N ratio is, the smaller the surface state density N_(FB) becomes,as shown in FIG. 1. It will be understood also that, for any certain O/Nratio, a smaller value of N_(FB) is obtained from the GaO_(x) N_(y) -GaAs system in which the deposition of the GaO_(x) N_(y) film is made ata lower temperature. It should be understood, however, that the specificresistance of the GaO_(x) N_(y) film will gradually become smaller witha lower O/N ratio, O/N≈0.2 serving as the boundary. As such, in case itis intended to form a GaO_(x) N_(y) film having a smaller O/N ratio,there appears a limit thereto from the aspect of electrical breakdownvoltage. This may be attributable to the fact that, in case O/N ratio isless than about 0.2, there arises a deterioration of adherency betweenthe GaO_(x) N_(y) film and the GaAs substrate, and that, accordingly,the quality of the film becomes poor, as shown in FIG. 2. However, inthe range of O/N ratio from nearly 0.2 to 0.3, it is possible to easilyobtain a GaO_(x) N_(y) film having a resistivity which is high enoughfrom practical viewpoint and to obtain those systems having a surfacestate density of 1×10¹¹ cm⁻² or lower. Accordingly, in case thesubstrate is GaAs, it is desirable to set O/N ratio at a value of atleast 0.15, and more desirably at least 0.2. The higher the O/N ratio isincreased, the higher will rise the specific resistance of the GaO_(x)N_(y) film, and in accordance therewith the surface state density N_(FB)also will increase. At the deposition temperature of 550° C. and whenthe O/N ratio is in the vicinity of 0.8, the surface state densityN_(FB) will take a value 0.8-1×10¹² cm⁻² which is substantially equal tothe N_(FB) value of conventional Ga₂ O₃ - GaAs systems. It should benoted here that the composition of GaO_(x) N_(y) film at O/N ratio of0.8 is about: Ga≈46 atomic %, 0≈24 atomic % and N≈30 atomic %, and thecomposition at O/N ratio of 0.2 is about: Ga≈48.5 atomic %, 0≈8.5 atomic% and N≈43 atomic %. As such, from the aspect of only surface statedensity, the present invention is able to freely encompass the rangefrom pure GaN up to Ga₂ O₃, although pure GaN and Ga₂ O₃ are outside therange of application of the present invention.

FIG. 3 shows an example of capacitance-voltage characteristic of anAu-GaO_(x) N_(y) - GaAs (p type) structure in case O/N ratio is 0.4 andthe GaO_(x) N_(y) film thickness is 2500 A. It should be understoodthat, on top of one surface of a p type GaAs substrate, a GaO_(x) N_(y)film of the present invention is formed. An electrode made of Au isprovided on this film. An ohmic contact made of an AgZn alloy (90:10 byweight) is provided on the other surface of this p type GaAs substrate.The frequency employed in the above-said capacitance-voltagecharacteristic test is 1 MHz. There is noted some hysteresis of thecarrier injection type, but this is negligibly small.

FIG. 4 shows the corresponding in-depth-composition distribution of theGaO_(x) N_(y) film-substrate structure which is used to measure the C-Vcurve as shown in FIG. 3, and this is a result obtained from theSputter-Auger method. In FIG. 4, the sputtering time (shown byhorizontal axis) corresponds to the depth or distance measured from theexposed surface of the film carried on the substrate. It will be easilyinferred from FIG. 4 that, in the vicinity of the interface between thefilm and the substrate, there is present only negligibly littletransition region of abnormal composition which is observed in knownthermal oxidation method or anodic oxidation method, and that thisinterface has little defects. From this Figure, it is clearly noted,therefore, that in the case of a GaO_(x) N_(y) film of the presentinvention, there can be materialized a satisfactory interface having alow surface state density.

Furthermore, from those results stated above, it is possible to obtain apassivation film having a small surface state density and exhibiting agood insulating ability, by establishing a relatively small O/N ratio ofa first GaO_(x) N_(y) film contacting the GaAs substrate to minimize thesurface state density of this film-substrate structure, and also byforming, on top of this film, a second GaO_(x) N_(y) film having a largeO/N ratio. The composition distribution of this structure is shown inFIG. 5, which is taken by using Auger electron spectroscopic analysiscombined with argon sputtering. The deposition temperature employed is450° C., and the supply source temperature is 320° C. The growth of thefirst layer which contacts the GaAs substrate is arranged so that theO/N ratio thereof is determined by the oxygen amount contained in thesupply source. At the time of growth of the second GaO_(x) N_(y) layer,the atmosphere gas is arranged to further contain nitrogen monoxide (NO)to thereby increase the amount of oxygen which is taken into the secondfilm. In this instance, the flow rate of the NO gas is 0.9 liter/min.for the rate of 2.5 liters/min. of NH₃ gas. In the case of FIG. 5, theO/N ratio of the first layer is about 0.3, and that of the second layeris about 0.77. It is preferable that the O/N ratio of the first layer isat least 0.15 and that the O/N ratio of the second layer is at least0.3. The surface state density resulting from this double layerGaO_(x).sbsb.2 N_(y).sbsb.2 -GaO_(x).sbsb.1 N_(y).sbsb.1 (wherein: x₂/y₂ ≧x₁ /y₁) which is formed continuously is a very small value, beingfor example 8×10¹⁰ cm⁻², and besides the insulating ability of this filmis good. It should be understood here in FIG. 5 that the relativelygentle variation noted in the composition located in the interfaceregion between the first GaO_(x) N_(y) layer and the substrate GaAs isattributable partially to the artifact arising from the sputteringevent, and thus in reality the thickness of such composition-varyingregion may be smaller than the width of the transition layer locatedbetween the first layer and the second layer.

Another method of forming a multi-layered film like the one stated aboveaccording to the present invention is based on the utilization of thefact that the O/N ratio in the GaO_(x) N_(y) film varies with thedistance L between the substrate and the supply source shown in FIG. 6.More particularly, this method utilizes the fact that the O/N ratio isgreat when this distance L is small, and that, conversely, the O/N ratiowill become small when the distance L is great, and thus a multi-layeredfilm having a composition distribution similar to that shown in FIG. 5can be formed. As an example, in case the substrate temperature is 500°C. and the supply source temperature is 280° C., and in case L=14 cm,the value of the O/N ratio obtained is 0.6-0.8, and in case L=22 cm, theO/N ratio obtained is 0.4-0.5. In this case, for the deposition of thefirst film the distance L is set large, and for that of the second filmthe distance L is set small. The reason for the variation in oxygencontent in association with the distance L is considered to be explainedas follows. The sub-oxide of gallium, such as Ga₂ O, which has a highequilibrium vapor pressure and which is contained in the source materialmay be deposited in greater amount onto the wall of the reaction tube incase the distance L is greater, and accordingly the amount of Ga and theamount of oxygen O which deposit on the surface of the substrate willdecrease.

According to this method described above, it is only necessary to varythe position of the substrate relative to the source, while keeping suchparameters as ambient gases and the source constant. Thus, this methodis very efficient to control the O/N ratio in the film. Furthermore, bycombining the above-stated method with those designed to vary the flowrate of the carrier gas and to vary the source temperature, the O/Nratio in the GaO_(x) N_(y) film can be varied more efficiently. Itshould be understood that in case the flow rate of the carrier gas isdecreased, the proportion of deposition of the gallium sub-oxide ontothe wall of the reaction tube increases, so that the O/N ratio willdecrease. Conversely, in case the supply source temperature is elevated,the supply rate of this source onto the substrate will relativelyincrease, so that O/N ratio will increase. It is needless to say thatthe number of the GaO_(x) N_(y) layers having different O/N ratios isnot limited to two layers mentioned above, and that a greater number oflayers may be provided.

Description has been made to those embodiments wherein the O/N ratio ofthe GaO_(x) N_(y) film is varied substantially stepwise in the directionof its thickness. However, the structure of the film may be of the typethat the O/N ratio of the GaO_(x) N_(y) film continuously increases asthe location goes closer toward the exposed surface thereof, as shown inFIG. 7. In this embodiment, O/N≈0.2 in the vicinity of the interface,and it is about 0.36 at the uppermost surface of the film.

Still another method of varying the O/N ratio in the film according tothe present invention is to subject the GaO_(x) N_(y) film prepared inadvance to heat treatment either in an ammonium gas atmospherecontaining an oxidizing gas or in a nitrogen gas atmosphere containing anitrogen oxide such as NO or NO₂. By conducting this heat treatment inan atmosphere containing nitrogen, it is possible to control the vaporpressure of nitrogen evaporating from the GaO_(x) N_(y) film, and nosuch abnormal oxide layer is produced at the interface as that noted inthe instance wherein oxidation is carried out in an atmospherecontaining only O₂ gas, and thus it is possible to uniformly oxidize allthe portions within the film. As an example, description will be madehereunder with respect to the instance wherein an NH₃ atmosphere isemployed. The composition distribution in the instance wherein heattreatment is conducted for 30 minutes at 550° C. by adding O₂ at therate of 100 cc/min. to the NH₃ of a flow rate of 1 liter/min. is shownin FIG. 8. The dotted line and the one-dot chain line indicate thecomposition distributions of oxygen and nitrogen, respectively, prior toheat treatment. It will be noted that only the GaO_(x) N_(y) film isoxidized, and that oxidation of that portion of the surface of the GaAssubstrate located at the interface has not taken place. Similarphenomena are noted also in the instance wherein a nitrogen oxide suchas NO or NO₂ is mixed in the N₂ atmosphere. It will be understood thatsuch methods are very effective as a means for controlling the O/N ratioin the GaO_(x) N_(y) film without sacrificing those interfacecharacteristics stated above.

Description will hereunder be made with respect to another method offorming GaO_(x) N_(y) films. An example of such method using theRF-reactive sputtering technique is shown in FIG. 9. From an RF(radio-frequency) oscillator of, for instance, 13.56 MHz via animpedance-matching circuit, an electric power of the order of 200 W issupplied across the opposing two electrodes. Either N₂ gas or NH₃ gas,which in some cases is diluted by inactive gases such as Ar and He, isintroduced into a vacuum chamber until the pressure of this gas reaches10⁻⁴ -10⁻² Torr to thereby produce plasma therein. At the same timetherewith, in order to control O/N ratio, O₂ gas or other oxidizing gassuch as NO and NO₂ is introduced also at an arbitrary rate. One of thetwo electrodes is made of a Ga metal which serves as Ga source and GaAswhich will serve as a substrate is mounted on the other one of theelectrodes. Those sputtered Ga atoms easily react with the nitrogen andoxygen which are being energized in the plasma, and the resultingmolecules deposit as a GaO_(x) N_(y) layer on the GaAs substrate. Thereaction between Ga, N and O takes place also when gallium atoms contactthe exposed surface of the GaAs substrate. In this way, it is possibleto form a GaO_(x) N_(y) film having an arbitrary O/N value. In case offormation of an AlO_(x) N_(y) film, such film may be formed in a mannersimilar to that stated above, by the use of aluminum source.

FIG. 10 diagrammatically shows a method of forming a GaO_(x) N_(y) filmby relying on the low-pressure plasma reaction technique utilizingplasma discharge by parallel flat plate electrodes. As the Ga-supplysource, a Ga halide such as GaBr₃ is employed. The rate of evaporationof GaBr₃ from this Ga-supply source is controlled by controlling thetemperature of this supply source at a value between 120° C. and 200° C.The evaporated Ga from the supply source is transported into a vacuumchamber, and this vapor is mixed with an oxidizing gas which, typically,is O₂ gas which is introduced therein through another gas inlet and alsomixed with N₂ or NH₃ gas, while maintaining the overall pressure at avalue between 10⁻¹ and 100 Torr to render them to plasma state, and thusa GaO_(x) N_(y) is deposited onto the substrate. During the formation ofthis film, it is possible to obtain a GaO_(x) N_(y) of high quality byelevating the temperature of the substrate up to about 300° C. Sameprocedure applies to the formation of AlO_(x) N_(y) films.

In FIG. 10, mention has been made of parallel flat plate electrodes asplasma-generation electrodes, for the convenience of explanation. It isneedless to say that the electrodes may be of the coil type orcylindrical type, and that accordingly the reaction chamber will takethe configuration such as horizontal type. It should be understood alsothat, instead of energizing the reaction gases into plasma state, theymay be used also as the ground state at a low pressure to obtain anoxynitride film having a uniform quality throughout the film.

Also, by causing evaporation of a pure nitride and a pure oxide byelectron bombardment in a vacuum of 10⁻⁶ or lower, and by thus causingdeposition of those substances onto a same surface of a substrate, it ispossible to form an oxynitride. In such instance, it is possible to varythe O/N ratio of the film either by independently controlling thecurrent of electron beam bombarding these substances, respectively, orby indepnedently controlling the respective flow rates of the substancesthrough chopper means.

It will be clear that, in case other insulating material such as SiO₂,Si₃ N₄, Al₂ O₃, P₂ O₅, B₂ O₃, Ga₂ O₃, BN, or AlN is deposited onto thoseGaO_(x) N_(y) films stated above, the effect and advantages of thepresent invention are not sacrificed at all.

Another feature of the present invention is found in that the filmsobtained according to the present invention are superior to known anodicoxide films and like known films in the ability to resist against theattack of chemical reagents. Anodic oxide films are easily dissolved insuch well-known acid solutions as HF, HCl and H₂ SO₄. However, theGaO_(x) N_(y) films obtained according to the present invention exhibitstrong resistivity to these acids, excepting that these films areaffected by HCl of 50° C. or higher. Also, these films of the presentinvention are resistive, at 50° C., to the well-known chemical etchingsolution of GaAs, i.e. H₂ SO₄ :H₂ O₂ :H₂ O=4:1:1 (by volume).Accordingly, these films can be used as a masking material for selectiveetching of GaAs semiconductors. Furthermore, the structure of thesefilms is stable against heat treatment when they are subjected to thosehigh temperatures employed in the manufacturing processes of GaAsdevices.

The GaO_(x) N_(y) films having controlled O/N ratios which are obtainedaccording to the present invention are effective in case such film isused, for example, as a gate-insulating film for active portions of aninsulated-gate type field effect transistor (IG-FET) shown in section inFIGS. 11A-11C. It is possible to set the threshold voltage V_(th) of theIG-FET at an arbitrary value by controlling O/N ratio, and thus it ispossible to substantially enlarge the freedom of designing.

FIG. 11A shows an embodiment wherein the GaO_(x) N_(y) films 1 and 2having such different O/N ratios as having a smaller value in the film 1than that in the film 2 are superposed one upon another. Also, as in thecase of FIG. 11B, it is also possible to freely control the surfacepotential of the transistor without deteriorating the interfacecharacteristic by continuously increasing the O/N ratio toward theinterface with gate electrode 3 in the GaO_(x) N_(y) film 7.Furthermore, as shown in FIG. 11C, even in case the first-formed GaO_(x)N_(y) film 8 is used in combination with another insulating film 9 madeof, for example, Si₃ N₄, Al₂ O₃, Ga₂ O₃ or SiO₂, the interfacecharacteristic between this GaO_(x) N_(y) film 8 and the semiconductorsubstrate 12 is not spoiled. Herein, reference numeral 3 represents agate electrode. 4 and 5 represent a source region and a drain region,respectively. 4' and 5' represent a source electrode and a drainelectrode, respectively. 6 represents a field insulating film made of,for example, SiO₂. 12 represents said semiconductor substrate. Thesource region 4 and the drain region 5 have a conductivity type oppositeto that of the semiconductor substrate 12.

Apart from the application of the present invention to IG-FET which hasbeen described above, an example wherein the present invention isapplied as a passivation film of pn junction is shown in FIGS. 12A and12B. Formation of a contact window through the GaO_(x) N_(y) film isperformed by relying on the ordinary photo-etching technique. As onesuch example, a photo-resist which is resistant to heat and acid, forexample, AZ 1350J (name of a product of Shipley Company of U.S.A.), isapplied onto the GaO_(x) N_(y) film 10 as a first step, followed byexposure to light and by development. Thereafter, the photo-resistlocating at such sites intended for electrodes is removed to expose theGaO_(x) N_(y) film thereat. Etching of the GaO_(x) N_(y) film is carriedout by the use of a hot phosphoric acid (H₃ PO₄) solution held at 70° C.The etching rate is substantially 100 A/min.. In order to improve theadherency between an ohmic contact metal and the substrate after theGaO_(x) N_(y) film has been locally removed, the surface of thesubstrate is subjected to slight etching by the use of an etchantsolution of, for example, H₂ SO₄ :H₂ O₂ :H₂ O=10:1:1 (by volume), tothereby expose a fresh surface of the substrate. On top of this portionof surface, there is placed an electrode-forming metal 11 by relying on,for example, evaporation technique and then subjected to sintering at anappropriate temperature and for an appropriate length of time. In FIGS.12A and 12B, reference numeral 13 represents an ohmic electrode foranother region. According to this method stated above, there can beobtained a pn junction having little leakage current and having a sharpbreakdown characteristic. It is needless to say that his surfacepassivation method can be applicable to all kinds of pn junctions and toSchottky junctions which are constructed by III-V compoundsemiconductors in such devices as laser diode, light-emitting diode,field effect transistor, static induction transistor and solar cell.

As stated concretely above, the GaO_(x) N_(y) films according to thepresent invention can be applied not only to all those technical fieldsin the manufacture of planar devices which have been fulfilled by SiO₂film (provided that, with respect to selective oxidation, theapplication thereto of the film of the present invention is difficultfrom the viewpoint of special nature of the thermal oxidation of theGaO_(x) N_(y) -III-V compound semiconductor systems), but also can beappropriately applied to particular fields of fabrication processes ofIII-V compound semiconductor devices such as encapsulating films forpost-ion implantation annealing.

Description has been made above with respect to embodiments wherein theoxynitride films are applied as passivation films of the furfaces ofsemiconductors. It should be understood that the oxynitride filmsaccording to the present invention are such that, by altering the O/Nratios thereof, the dielectric constant and the refractive index of thefilms can be altered also. As one such example, variation of therelative dielectric constant due to changes in the O/N ratio of theGaO_(x) N_(y) film is shown in FIG. 13. Within the range of O/N ratio ofabout 0.5 or higher, the value of relative dielectric constant willdecrease continuously with an increase in the O/N ratio, and willgradually approach the value of about 3.1 of Ga₂ O₃. Such nature of thefilm is very useful as an optical material in, for example,anti-reflection coating.

As explained above, the object of the present invention lies in theprovision of gallium oxynitride films, aluminum oxynitride films andmixture films of these two substances, and also in the provision of amethod of making these films. By utilizing the fact that the opticalconstant of such film as mentioned above can vary by varying the ratiobetween oxygen and nitrogen to be contained in the film, the film may beused also as a reflection-proof film, this implying the effectiveness ofthe oxynitride film in its application to an optical material.Similarly, the film may be utilized as a dielectric material, andfurthermore, as a surface passivation film for III-V compoundsemiconductors. It should be understood that, in case of GaO_(x) N_(y)film, the surface state density can be made low, and along therewith theinsulating ability of the film can be enhanced either by uniformalizingthe composition distribution of the film or by making small the O/Nratio of the first GaO_(x).sbsb.1 N_(y).sbsb.1 film (layer) whichcontacts a III-V compound semiconductor substrate, and by arranging soas to stepwisely or continuously increase the O/N ratio of theadjacently disposed second GaO_(x).sbsb.2 N_(y).sbsb.2 film (layer).Thus, the film is most suitable for surface passivation ofinsulated-gate type field effect transistor and static inductiontransistor, laser diode, light-emitting diode, solar cell and variousother electronic devices, and therefore, the industrial value of thesefilms of the present invention is very great.

What is claimed is:
 1. A Group III-V semiconductor substrate wherein thesemiconductor is formed with at least one element of the Group IIIelements and at least one element of the Group V elements in theperiodic table, said substrate having a main surface;a film formed onsaid substate and substantially formed of oxynitride of at least onesubstance selected from the group consisting of gallium, aluminum andtheir mixture, said oxynitride having a finite O/N ratio by atomicfraction greater than 0.15.
 2. A combination according to claim 1,further comprising: a conductive layer provided at least locally on saidfilm, that side of said film located opposite to the side contactingsaid main surface, said substrate comprising a pair of heavily dopedregions exposed at said main surface and partially contacting said film,said regions having conductivity type opposite to that of saidsubstrate.
 3. A combination according to claim 1, in which: saidsubstrate comprises at least one pn junction terminating at said mainsurface covered with said film.
 4. A combination according to claim 1,in which: said substrate comprises at least one boundary between spacecharge region and neutral region, said boundary terminating at said mainsurface covered with said film.
 5. A combination according to claim 1,further comprising at least one layer of insulating film other than saidoxynitride film of gallium, aluminum, or their mixture.
 6. A combinationaccording to claim 1, in which: said film is formed at least locally onthe main surface of said group III-V semiconductor substrate and has anon-uniform composition distribution that the O/N ratio by atomicfraction in that side of the film contacting the main surface of saidsubstrate is small and that this ratio increases at the region locatedwithin thickness of said film and being apart from said side.
 7. Acombination according to claim 6, in which: said O/N ratio by atomicfraction is at least 0.2.
 8. A combination according to claim 6, inwhich: said O/N ratio by atomic fraction is at least 0.3.
 9. Acombination according to claim 6 in which: the O/N ratio by atomicfraction of said region is at least 0.3.
 10. A combination according toclaim 6 further comprising a conductive layer at least locally on thatside of said film located opposite to the side contacting the mainsurface of said substrate.
 11. A combination according to claim 1, inwhich: said film is formed at least locally on the main surface of saidGroup III-V semiconductor substrate and has a non-uniform compositiondistribution that the O/N ratio by atomic fraction in that side of thefilm contacting the main surface of the substrate is small and that thisratio increases continuously in such a manner through the thickness ofsaid film that said ratio takes a minimal value at said side.
 12. Acombination according to claim 11, in which: said O/N ratio by atomicfraction is at least 0.2.
 13. A combination according to claim 11, inwhich: said O/N ratio by atomic fraction is at least 0.3.
 14. Acombination according to claim 1, further comprising a conductive layerformed at least locally on that side of said film located opposite tothe side contacting the main surface of said substrate.
 15. Acombination according to claim 14, further comprising at least one layerof insulating film other than said oxynitride film of gallium, aluminum,or their mixture, at a site located between the conductive layer andsaid oxynitride film.
 16. A combination according to claim 1 wherein thesemiconductor is formed with at least one of the elements gallium,aluminum, and indium and at least one of the elements arsenic andphosphorus.
 17. A combination according to claim 16 wherein thesemiconductor is GaAs.
 18. A combination according to claim 16 whereinthe semiconductor is Ga_(x) Al_(1-x) As.
 19. A combination according toclaim 16 wherein the semiconductor is GaAs_(x) P_(1-x).
 20. Acombination according to claim 16 wherein the semiconductor is In_(x)GA_(1-x) As_(y) P_(1-y).